Atrial fibrillation is a form of arrhythmia characterized by rapid, uncoordinated and uncontrolled contractions of the atria (the upper chambers of the heart). While typically not fatal, atrial fibrillation keeps the heart from pumping blood efficiently and is a common risk factor for, and cause of, stroke. Atrial fibrillation also markedly diminishes quality of life, and exacerbates heart failure. In the United States, atrial fibrillation presents an immense health care and financial burden, affecting over 2 million people with annual medical expenditures exceeding billions of dollars. The prevalence of atrial fibrillation increases strikingly with advancing age, in that 4% of men and women in the U.S. older than 60 and 10% of those older than 80 suffer from atrial fibrillation.
Traditional approaches to atrial fibrillation treatment centered primarily on pharmacological therapy. A more recent non-pharmacological technique for preventing or terminating arrhythmias is to overdrive pace the heart wherein an implantable cardiac stimulation device, such as a pacemaker, applies electrical pacing pulses to the heart at a rate somewhat faster than the intrinsic heart rate of the patient. To prevent tachyarrhythmias from occurring, the cardiac stimulation device artificially paces the heart at a rate of, generally, five to ten pulses per minute faster than the intrinsic heart rate of the patient. This technique is often referred to as Dynamic Overdrive Pacing or DAO. In other words, a slight artificial tachycardia is induced and maintained in an effort to prevent an actual tachycardia from arising. If an actual tachycardia occurs, the cardiac stimulation device senses the tachycardia and immediately begins pacing with short bursts at a rate of about 10–20% faster than the tachycardia in an effort to capture the heart and thereby terminate the tachycardia. This technique is often referred to as Anti-Tachycardia Pacing or ATP.
It is believed that overdrive pacing is effective for at least some patients for preventing the onset of an actual tachycardia for several reasons. A normal, healthy heart beats only in response to electrical pulses'generated from a portion of the heart referred to as the sinus node. The sinus node pulses are conducted to the various atria and ventricles of the heart via certain, normal conduction pathways. In some patients, however, additional portions of the heart also generate electrical pulses referred to as “ectopic” pulses. Each pulse, whether a sinus node pulse or an ectopic pulse has a refractory period subsequent thereto during which time the heart tissue is not responsive to any electrical pulses. A combination of sinus pulses and ectopic pulses can result in a dispersion of the refractory periods, which, in turn, can trigger a tachycardia. By overdrive pacing the heart at a uniform rate, the likelihood of the occurrence of ectopic pulses is reduced and the refractory periods within the heart tissue are rendered uniform and periodic. Thus, the dispersion of refractory periods is reduced and tachycardias triggered thereby are substantially avoided.
It is therefore desirable within patients prone to tachyarrhythmias to ensure that most beats of the heart are paced beats, as any unpaced beats may be ectopic beats. A high percentage of paced beats can be achieved simply by establishing a high overdrive-pacing rate. But, a high overdrive-pacing rate is disadvantageous in that it may be unpleasant to the patient, particularly if the artificially induced heart rate is relatively high in comparison with the heart rate that would otherwise normally occur. A high heart rate may also cause possible damage to the heart or may possibly trigger more serious dysrhythmias, such as a ventricular fibrillation. A high overdrive-pacing rate may be especially problematic in patients suffering from heart failure, particularly if the heart failure is due to an impaired diastolic function. A high overdrive-pacing rate may actually exacerbate heart failure in these patients. Also, a high overdrive-pacing rate may be a problem in patients with coronary artery disease because increasing the heart rate decreases diastolic time and decreases perfusion, thus intensifying ischemia. Also, the need to apply overdrive-pacing pulses operates to deplete the implantable cardiac stimulation device's internal power supply, perhaps requiring frequent surgical replacement of the power supply.
In view of these concerns, a desirable approach to overdrive pacing is to reduce the average overdrive-pacing rate, while still attaining a sufficiently high rate to significantly reduce the likelihood of a dysrhythmia within the patient or to terminate a dysrhythmia if one nevertheless occurs. Such an overdrive pacing technique permits a certain percentage of paced beats (such as 90% or 95%) to be sustained by the cardiac stimulation device so as to enable the overdrive pacing rate to be minimized while ensuring that most beats of the heart are paced beats. An example of an overdrive pacing system that operates in this manner to reduce the likelihood of a dysrhythmia within a patient, or terminate a dysrhythmia altogether, is provided by U.S. patent application Ser. No. 09/471,788, for “Methods and Apparatus for Overdrive Pacing Heart Tissue Using an Implantable Cardiac Stimulation Device,” incorporated by reference herein.
Another approach to treatment of arrhythmias is to pace within multiple sites of the atria. Such multi-site atrial pacing has been used to suppress atrial fibrillation in patients with sinus node disease or that have been rendered bradycardiac by antiarrhythmic drugs used to maintain sinus rhythm. Some studies also suggest that multi-site pacing is an effective method of preventing paroxysmal atrial fibrillation by increasing cardiac performance through electrical resynchronization. It would therefore be desirable to provide techniques that combine beneficial aspects of overdrive pacing and multi-site atrial pacing in order to provide improved treatment for atrial fibrillation. It is to this end that aspects of the present invention are primarily directed.